Here's how you write such a deal: "As of date X, all existing EU regulations remain in force. Changes to UK law that overlap with existing areas of EU regulati

Here's how you write such a deal: "As of date X, all existing EU regulations remain in force. Changes to UK law that overlap with existing areas of EU regulation will result in a notification to the Commission". Done. There is no need to 'replace' detailed regulation on a thousand topics when, as you observe, it is already implemented in UK law.